1. Field of the Invention
The present invention relates to a developing apparatus that performs a development using a mono-component developer, and is preferably applicable to an image forming apparatus of a laser beam printer, a copying machine, and the like, that forms images using an electrophotographic system or an electrostatic recording system.
2. Description of the Related Art
An electrophotographic image forming apparatus of a copying machine, a laser beam printer, and the like, forms an electrostatic image (latent image) by irradiating an electrophotographic photosensitive member (photosensitive member) with a light corresponding to image data. A toner of a developer as a recording material is supplied to the electrostatic image from a developing apparatus, to develop a toner image. The toner image is transferred from the photosensitive member to a recording material such as a recording paper by a transfer device. The toner image is fixed on the recording material by a fixing device, thus forming a recorded image.
A variety of apparatuses are proposed regarding a developing apparatus using a dry mono-component developing process. There is one in the following as an example. That is, a magnetic mono-component developer (magnetic toner) is carried on a developing sleeve serving as a developer carrying member, and a uniform toner layer is formed by a layer thickness regulating member. The developing sleeve is caused to be in close proximity to or come into contact with a photosensitive member. Then, a developing bias voltage composed of alternating current and direct current components, for example, is applied to the developing sleeve. As a result, an electric potential is generated between the electrostatic image on the photosensitive member and the developing sleeve. Thereby, toner is moved to the electrostatic image to perform a development.
To further describe, such a developing apparatus has a cylindrical-shaped developing sleeve provided rotatably at an opening of a developing container that contains a magnetic toner. Within the developing sleeve, there is provided a magnetic field generating unit (magnet roller) having a plurality of securely arranged magnetic poles. The magnetic toner is adsorbed onto the developing sleeve by the magnetic field generated by the magnetic field generating unit, and thereby the toner is carried on the developing sleeve and conveyed. Also, in such a developing apparatus, a toner layer is formed on the developing sleeve by a developer regulating member that abuts on the developing sleeve. As the developer regulating member, a blade-shaped member formed generally by an elastic body, that is, a developing blade is used.
In recent years, an enhancement of resolution, definition of images has been demanded, thus conglobation and reductions in the diameter of the toner used for the developing apparatus has progressed. In particular, a conglobated toner has been widely used because it has a high electrostatic charge amount per weight Q [μC/g], which effectively contributes to an improvement of reproducibility of dot image or fine-line image, and to an improvement of transferring property.
However, when the conglobated toner is used, the following problems may arise in some cases.
That is, regarding a toner with high sphericity, there is a tendency that a toner conveyance amount M [g/m2] passing through the developing blade on the developing sleeve and conveyed to a developing region increases. In particular, the tendency appears during printing at a low coverage rate (output of images with low image ratio), or after idle rotation.
Then, there are cases where an excessive increase in a toner conveyance amount generates dispersion in the distribution of electrostatic charge amount of toner, induces an uneven to a toner coat on the developing sleeve, and causes an uneven image density to develop.
Also, an increase in the toner conveyance amount is likely to impart insufficiently an electrostatic charge to the toner between the developing sleeve and the developing blade. Thus, in some cases, conveyance of the insufficiently charged toner to a developing region causes, so called, a fogged image to occur in which the toner adheres to portions other than electrostatic latent images on the photosensitive member (non-image portions).
It was found that this tendency is peculiar, in particular, in a developing apparatus using a magnetic mono-component developer (magnetic toner). This is attributable largely to that, since the toner is carried by a magnetic force of a magnet within the developing sleeve, there is no action to brush off toner residue after developing by a feed roller like a developing apparatus using a non-magnetic mono-component developer (non-magnetic toner).
That is, this is probably because the toner residue after developing is not brushed off from the developing sleeve, but is coated on the developing sleeve together with a newly fed toner, and thus the toner coat becomes unstable.
As a unit to suppress the increase in a toner conveyance amount M [g/m2], as described above, conventionally, a control has been performed mainly by a combination of methods (α) to (γ) as below.
(α) To reduce a surface roughness [μm] of the developing sleeve;
(β) To increase an abutment pressure P [g/cm] of the developing blade to the developing sleeve;
(γ) To shorten a distance (hereinafter referred to as “NE length”) [mm] from an abutment position between the developing blade and the developing sleeve to a free end of the developing blade.
That is, any of methods (α) to (γ) is a method for regulating a toner conveying force mechanically and there is a limit depending on a manufacturing dispersion or a mounting dispersion. Also, an increase in an abutment pressure P [g/cm] causes a mechanical stress to the toner to increase, accelerates deterioration of the toner, and thus leading to reduction in the image density. Also, when a surface roughness of the developing sleeve is set low, the durability drops, and thus it is disadvantageous for higher-speed/longevity of the image forming apparatus.
Also, a variety of toner layer forming technologies are proposed.
There is a proposal relating to a developer regulating member, in which fluctuations of toner conveyance amount are suppressed over a long period of use by specifying surface roughness Ra [μm] of a soft elastic body serving as a layer forming member and curvature radius of concave portion (Refer to Japanese Patent Application Laid-Open No. 62-242975).
Also, there is another proposal, in which a uniform thin layer/an increase in electrostatic charge amount of magnetic mono-component developer (magnetic toner) are aimed at by specifying surface roughness Rz of a toner regulating member, and high-quality of images in initial use condition is aimed at (Refer to Japanese Patent Application Laid-Open No. 2004-117919).
Also, there is still another proposal, in which a uniform thin layer on an elastic developing roller and prevention of images from deteriorating after leaving them for a long period of time are aimed at by specifying surface roughnesses Ra, Rz, and Rmax of a developer regulating member (Refer to Japanese Patent Application Laid-Open No. 2004-12542).
As described above, a method for regulating roughness of surface of a developer regulating member with the aim to obtain a stable toner layer is an effective method. However, depending on surface shape of the developer regulating members, external additive agent liberated from the toner may become clogged into concave portions of the surface of the developer regulating member, and streak images may occur due to clogging.
As an external additive agent of toner, silica is named, for example. Also, particle diameter of the silica to be used for the external additive agent of toner is commonly in the order of 1 to 100 nm.
The above-mentioned streak image is more likely to occur particularly, since the external additive agent of toner is adapted to be easily liberated under a high-temperature high-humidity environment.
Also, the higher the rotational speed of a developer carrying member, namely, a developing sleeve, the more likely streak images tend to occur, since the external additive agent of toner is adapted to be easily liberated.
Also, the more the total number of rotations of a developing sleeve becomes, the more the amount of external additive agent that has been liberated from the toner becomes, thus the more likely streak images tend to occur.
Under such circumstances, there is a yet another proposal, in which, in order to prevent a developer from fixing to a developer regulating member, a resin coating layer is formed on the surface of the developer regulating member, and surface roughnesses Ra [μm] of the developer regulating member and a developer carrying member are specified (Refer to Japanese Patent Application Laid-Open No. 6-186838).
As described above, it was found that it is difficult to satisfy both of stability of developer layer thickness and prevention of streak images resulting from the clogging of external additive agent of developer into concave portions on the surface of the developer regulating member, only by paying attention to Ra, Rz, and Rmax regarding the surface roughnesses of the developer regulating member and the developer carrying member.